Microspheres from light—a sustainable materials platform

Driven by the demand for highly specialized polymeric materials via milder, safer, and sustainable processes, we herein introduce a powerful, purely light driven platform for microsphere synthesis – including facile synthesis by sunlight. Our light-induced step-growth precipitation polymerization produces monodisperse particles (0.4–2.4 μm) at ambient temperature without any initiator, surfactant, additive or heating, constituting an unconventional approach compared to the classically thermally driven synthesis of particles. The microspheres are formed via the Diels-Alder cycloaddition of a photoactive monomer (2-methylisophthaldialdehyde, MIA) and a suitable electron deficient dienophile (bismaleimide). The particles are stable in the dry state as well as in solution and their surface can be further functionalized to produce fluorescent particles or alter their hydrophilicity. The simplicity and versatility of our approach introduces a fresh opportunity for particle synthesis, opening access to a yet unknown material class.

1. The manuscript mainly deals with the synthesis of microspheres, which is of course a relevant topic. Nevertheless, I miss a critical discussion on the impact of the work.
2. How dispersible are these particles and how stable are dispersions?
3. The authors use remaining maleimide functions to functionalize the particle surface. Did the authors think about addition of functional groups to increase dispersibility or to introduce other properties? I would expect a bit more on this topic for a publication in Nat. Commun.
4. It seems like the particles/polymers are not soluble. Which solvents were tested to solubilize the particles? 5. Do the authors have an idea about polymerization conversion? 6. The authors discuss thermal stability of the particles. I would suggest to perform DSC and maybe TGA to support the statements. 7. Did the authors think about probing the porosity of the particles? 8. In Figure 1, nucleation and particle growth are illustrated, yet nucleation is not described in the manuscript. I suggest to follow the particle growth in time. It might be even useful to see whether nanaoparticle are accessible as well. Figure S11 CDCl3 should be CHCl3, on page 2 it should be the CDCl3 triplet.

10.
page 12 in the SI "150C" 11. superscripts in the SI should be checked Reviewer #2: Remarks to the Author: Barner-Kowollik et al have produced new polymeric particles by a light induced precipitation polymerisation. Here, the simple light driven Diels-Alder coupling of bismaleimide with photoactivated dialdehydes yields a polymer that precipitates to form polymeric particles. This step-growth polymerisation can be undertaken under mild conditions and produces monodisperse particles due to the mechanism of formation. However, the targeted application of these particles is still unclear. The particles have maleimide functionality on the surface (although this isn't quantified) which enables a range of different species to be attached to the surface. The authors have chosen to do a NITEC reaction to create fluorescent particles. This, however, does not require the mild conditions and surfactant free environment that makes these particles unique. Although I believe the new method and particles synthesised are of interest, I think the application currently lets them down. Further to this I have a few questions regarding the rest modification and stability tests.
 Can the authors comment on the colloidal stability of the particles after modification. E.g. SEM/DLS before and after?  Do the authors have an idea of how much of surface they can functionalise using the NITEC reaction. 50% of the surface should be maleimide therefore I would expect a much higher fluorescence intensity to be achieved.  Is the stability test of the particles conducted with the modified particles? o If so, then please make this clearer. o If the stability tests are conducted before modification, I would suggest that the manuscript is reordered to put the functionalisation at the end. This would make the paper easier to read and follow.

Reviewer #2
Barner We thank the reviewer for their comments and we have amended the manuscript to highlight the versatility of our new method. Furthermore, we performed a thiol-ene reaction with the residual maleimide and a poly(ethylene glycol) methyl ether thiol (2000 g mol -1 ) to allow the dispersibility of the particles in water. Both the NITEC and thiol-ene reactions were carried out at ambient temperature without any surfactant and the particles are recovered by simple centrifugation. We believe that these post-functionalization approaches are mild and open several avenues to tailor our particles to specific applications, e.g., in point-of-care applications or as chromatography materials.
Comment 1: Can the authors comment on the colloidal stability of the particles after modification.

E.g. SEM/DLS before and after?
Reply: We thank the reviewer for raising that point. The particles are stable after modification with the tetrazole and PEG-SH (added new experiment to demonstrate the impact of pegylation) and we have added pictures for visual inspection (Figure 6 and Figure S12). We further added SEM pictures before and after the modification step as well as the corresponding particles sizes in Figure S14 and Table S3.
Comment 2: Do the authors have an idea of how much of surface they can functionalise using the NITEC reaction. 50% of the surface should be maleimide therefore I would expect a much higher fluorescence intensity to be achieved.

Reply:
We acknowledge the reviewer's comment, yet it is difficult to quantify the percentage of residual maleimide. As per step-growth polymerization's mechanism, the 1:1 stoichiometry of aldehyde and maleimide functional groups is not retained on the surface: the bismaleimide will react to form the polymer chain and the residual maleimide moieties on the surface are only associated with unreacted polymer chain ends. As the proportion of end chains is very low, surface-sensitive methods such as X-ray Photoelectron Spectroscopy or Time-of-Flight Secondary Ion Mass Spectrometry are analytically challenging in this case, and we therefore opted to qualify the presence of maleimide by chemical transformation.

Comment 3: Is the stability test of the particles conducted with the modified particles?
o If so, then please make this clearer.
o If the stability tests are conducted before modification, I would suggest that the manuscript is reordered to put the functionalisation at the end. This would make the paper easier to read and follow.

Reply:
We thank the reviewer for raising that concern. The stability tests were conducted on the non-modified particles. We have thus re-ordered the manuscript as suggested with the stability presented first and the functionalization next. We also have re-numbered the figures accordingly.